While not impossible in the future, there's no focus on our ARMA III server right now to become highly populated. 77th JSOC, au contraire, runs an entire clan ecosystem around Arma with events, many active admins etc. They have a very nice, teamwork-based server like Blizz pointed out.
Do note though that 77th JSOC is very strict. Literally waiting in line, strict hierarchy, and certain commanders are even such doofuses that they won't reply to you unless you address them in the exact way they want . That said, there's an amazing grade of teamwork to be found. The 77th JSOC servers always run Invade And Annex, which is a mission system that spawns a main and side objectives. The main objective (Area Of Operations) has a few tactical targets and enemies. Take them down and the AO is over, and it will respawn in a new place shortly after.
While Arma 3 has a VAST range of vehicles, you won't use many on that server. It's highly infantry-tailored gameplay. There are 4 helicopters, 1 jet, and 1 armored unit max. Armor can only serve as support and must stay out of the AO until called in by inf, which RARELY ever happens. Command can flat-out refuse to issue armor squad and then you're out of luck.
It's a very fun way of play in itself, but bear in mind you will be waiting and walking for very long times. Depending on the commander, it's not uncommon to wait 20 minutes between deployments or to be stuck in base for half an hour before you can be sent out. If this is your style of play though, there's no better game and place than Arma 3 and the 77th JSOC at the moment.